1. Field of the Invention
This invention relates to a method of reducing a contact impedance between a gate electrode of a thin film transistor and a Indium Tin Oxide (ITO).
2. Description of the Prior Art
Nowadays, typical cathode ray tubes (CRTs) are made in larger size to provide users with better visual effect. Similarly, liquid crystal displays (LCDs) are also made as large in size as possible.
The LCD has the inherent advantages of low power consumption and less weight, over the CRT. Specifically, for those CRT products that are over 38″ inches, they are much heavier than comparable LCD products. Further, large size CRT's are also more difficult to manufacture. Therefore, the LCD product has clearly become the better choice for customers in purchasing large-sized displays.
However, large size (more than 20″) LCDs having a high resolution need a shorter RC delay time. In those conventional arts, materials of a thin film transistor (TFT) on a LCD are Al, Cr, W, Ta, Mo and so on. Because of AL having a higher electric conductivity (i.e.: the resistance R is lower), a gate electrode of the TFT is generally made of single-layer Al or double-layer of Al/Al alloy or Al alloy/other metals. For single layer Al, a layer of oxide will be generated on the surface of Al due to Al contacting air when the Al is moved from a vacuum chamber to proceed next step. The oxide layer can not be cleaned away with an etching liquid and so when Al layer is connected with other metal, the contact impedance between the Al layer and the other metal is very high. On the other hand, the double-layer structure made of a first metal layer 20 and a second metal layer 30 (e.g.: Al/Al alloy or Al alloy/other metals), as shown in FIG. 1 has difference etching rates resulted from different metal. Therefore an eaves structure will occur after etching process due to the two layers having different etching rate. Namely, two edges of the connecting part between the first metal 20 and the second metal 30 as shown in FIG. 1 have bulges. Therefore, some etched residual impurities will easily remain under the bulges and be hard to clean away. The following processes are influenced and the contact impedance and the impedance of the gate electrode of double-layer becomes bad. If the gate electrode is made of single-layer Cr or other metal, the contact impedance between the gate electrode and ITO is small but the impedance of the gate electrode is higher and so it can not achieve a shorter RC delay time.
Hence, the development of large size LCD must solve the problem of reducing the gate impedance and the contact impedance between the gate electrode and the other metal of the TFT.